An evaluation of the inorganic and organic geochemistry of the San VicenteMississippi Valley-type zinc-lead district, central Peru: Implications forore fluid composition, mixing processes, and sulfate reduction

Mississippi Tialley-type zinc-lead deposits and ore occurrences in the San Vicente belt are hosted in dolostones of the eastern Upper Triassic to Lowe r Jurassic Pucara basin, central Peru. Combined inorganic and organic geoch emical data from 22 sites, including the main San Vicente deposit, minor or e occurrences, and barren localities, provide better understanding of fluid pathways and composition, ore precipitation mechanisms, Eh-pH changes duri ng mineralization, and relationships between organic matter and ore formati on. Ore-stage dark replacement dolomite and white sparry dolomite are Fe an d rare earth element (REE) depleted, and Mn enriched, compared to the host dolomite. In the main deposit, they display significant negative Ce and pro bably Eu anomalies. Mixing of an incoming hot, slightly oxidizing, acidic b rine (H2CO3 being the dominant dissolved carbon species), probably poor in REE and Fe, with local intraformational, alkaline, reducing waters explains the overall carbon and oxygen isotope variation and the distributions of R EE and other trace elements in the different hydrothermal carbonate generat ions. The incoming ore fluid flowed through major aquifers, probably basal basin detrital units, with limited interaction with the carbonate host rock s. The hydrothermal carbonates show a strong regional chemical homogeneity, indicating access of the ore fluids by interconnected channelways near the ore occurrences. Negative Ce anomalies in the main deposit, that are absen t at the district scale, indicate local ore-fluid chemical differences. Oxi dation of both migrated and indigenous hydrocarbons by the incoming fluid p rovided the local reducing conditions necessary for sulfate reduction to H2 S, pyrobitumen precipitation, and reduction of Eu3+ to Eu2+. Fe-Mn covariat ions, combined with the REE contents of the hydrothermal carbonates, are co nsistent with the mineralizing system shifting from reducing/rock-dominated to oxidizing/fluid-dominated conditions following ore deposition. Sulfate and sulfide sulfur isotopes support sulfide origin from evaporite-derived s ulfate by thermochemical organic reduction; further evidence includes the p resence of C-13-depleted calcite cements (similar to-12 parts per thousand delta(13)C) as sulfate pseudomorphs, elemental sulfur, altered organic matt er in the host dolomite, and isotopically heavier, late, solid bitumen. Sig nificant alteration of the indigenous and extrinsic hydrocarbons, with abse nt bacterial membrane biomarkers (hopanes) is observed. The light delta(34) S of sulfides from small mines and occurrences compared to the main deposit reflect a local contribution of isotopically light sulfur, evidence of loc al differences in the ore-fluid chemistry.